In certain applications, there is a need to broadcast a common signal to multiple locations on an IC chip. A control signal that operates multiple switches is one example of a signal that might be broadcast to multiple locations. A more common example is an electronic clock for synchronizing multiple devices on the chip. Yet another common example is the use of broadcast distribution of information on a signal bus, where one location distributes signals to many locations in a broadcast mode.
Electronic clocks are limited by timing jitter from variations in the electrical transmission of the high bandwidth signals. This limitation is expected to be a serious detriment to performance in microprocessors as they approach clock speeds around or above 20 GHz. Also, on large microprocessor chips, the distribution of electronic clock signals can account for up to 5–15% of the total heat generated by the chip.
Distributing the clock as an optical signal could solve some of the more serious problems that are presented by distributing the clock as an electrical signal. The use of optical clocks offers the potential of helping enable the continued growth in microprocessor speed and achieving operational efficiency if a means can be found for implementing the optical clock in a production process at low cost and high efficiency.
The combination of silicon and SiGe alloy (e.g. SixGe1-x) has attracted attention as useful combination of materials from which one might be able to easily and economically fabricate optical signal distribution networks. With SixGe1-x it is possible to fabricate waveguides in the silicon substrates. The index of refraction of Si0.95Ge0.05 is slightly higher than that of silicon. For example, at a wavelength of 1300 nm to which silicon is transparent, Si0.95Ge0.05 with 5% Ge has a index of refraction of about 3.52 while crystalline silicon has an index of refraction that is less than that, e.g. about 3.50. So, if a SixGe1-x core is formed in a silicon substrate, the difference in the indices of refraction is sufficient to enable the SixGe1-x core to contain an optical signal through internal reflections. Moreover, this particular combination of materials lends itself to the use of conventional silicon-based semiconductor fabrication technologies to fabricate the optical circuitry.
The detectors that are most frequently considered for use in such optical signal distribution networks are low cost silicon-based detectors. Using silicon-based detectors, however, presents a fundamental challenge. Since the waveguides are made of silicon (i.e., SiGe), the wavelengths that are used for the optical signal must pass through the silicon without being absorbed. That is, the silicon must be transparent to those wavelengths. But to work as a detector, the silicon-based device must absorb that wavelength to be able to convert it into an electrical signal. Thus, such silicon-based detectors are typically characterized by a low-absorption efficiency. This typically means very high optical power levels must be used to create sufficient photocurrent to drive the detectors at the speeds required which means expensive lasers must be used.